In a typical hydrogen liquefaction plant, high pressure hydrogen gas (e.g., 15-70 bara) is purified and dried and sent to a cold box where it is cooled to approximately −190° C. Refrigeration for this level of cooling is typically provided by a closed loop nitrogen refrigeration cycle. The nitrogen refrigeration cycle may include a single turbine, multiple turbines, tubine(s) with booster(s)in addition to mechanical refrigeration unit utilizing ammonia or other refrigerant. Additionally, the nitrogen refrigeration cycle typically employs a multistage nitrogen recycle compressor to complete the closed loop.
Alternatively, for some applications this level of refrigeration (to −190° C.) is provided by injecting a stream of liquid nitrogen (LIN) into the exchanger at approximately −190° C. This nitrogen stream vaporizes and is warmed to near ambient temperature as it exchanges cold with the hydrogen streams which are being cooled. This alternative is less thermodynamically efficient and requires liquid nitrogen to be sourced from a separate nitrogen liquefier which would still require a cycle compressor and turbine boosters.
The cooled gaseous hydrogen is further cooled and liquefied at approximately −252° C. by a second refrigeration cycle. Refrigeration for this level of cooling can be provided by a closed hydrogen (or helium) refrigeration cycle with multiple turbines and a hydrogen (or helium) recycle compressor. This hydrogen (or helium) compression is very difficult and expensive because of the low molecular weight (MW) or more specifically because these molecules are so small.
Those of ordinary skill in the art will also recognize that production of liquid hydrogen requires other steps (e.g., adsorption systems, ortho—para conversion) which are not described herein as they are not impacted by embodiments of the current invention.
U.S. Pat. No. 2,983,585 (Smith) discloses a partial oxidation process in which methane is partially oxidized with oxygen to produce carbon monoxide and hydrogen gas. The partial oxidation process is integrated with a hydrogen liquefaction process in which hydrogen gas is pre-cooled by indirect heat exchange against liquid methane and subsequently further cooled against a closed external refrigerating cycle using liquid nitrogen (“LIN”) as the refrigerant. The resultant methane is compressed at the warm end of the liquefaction process and then fed to the partial oxidation process. The resultant gaseous nitrogen is compressed at the warm end of the closed cycle before being condensed by indirect heat exchange with liquid methane and recycled. It is disclosed that the liquid methane could be replaced with liquefied natural gas (“LNG”). However, with this scheme this warm end refrigeration load is simply shifted from the hydrogen liquefier unit to the natural gas liquefaction unit. An additional heat exchange system between nitrogen and LNG is required incurring additional thermodynamic losses. Also, the hydrogen stream is only cooled to approximately −150° C. due to the liquefaction temperature of LNG.
U.S. Pat. No. 3,347,055 (Blanchard et al.) discloses a process in which a gaseous hydrocarbon feedstock is reacted to produce hydrogen gas which is then liquefied in an integrated liquefaction cycle. In one embodiment, the liquefaction cycle involves two closed refrigerant cycles, the first using hydrogen gas as a refrigerant and the second using nitrogen. Compression for both refrigeration cycles takes place at the warm end of the cycles. The hydrogen to be liquefied is also cooled by indirect heat exchange against a liquefied hydrocarbon feedstock gas thereby producing gaseous feedstock at 1 atm. (e.g., about 0.1 MPa) for use in the hydrogen production plant. It is disclosed that the hydrocarbon feedstock may be natural gas. This scheme also is shifting part of the refrigeration load from the hydrogen liquefier to the natural gas liquefier.
JP-A-2002/243360 discloses a process for producing liquid hydrogen in which hydrogen that is similar to U.S. Pat. No. 3,347,055 Blanchard, feed gas is pre-cooled by indirect heat exchange against a stream of pressurized LNG. The pre-cooled hydrogen gas is fed to a liquefier where it is further cooled by indirect heat exchange against both LIN and a refrigerant selected from hydrogen or helium. The further cooled hydrogen is then expanded to produce partially condensed hydrogen which is separated into liquid hydrogen, which is removed and stored, and hydrogen vapor which is recycled in the liquefier.
Quack discloses (“Conceptual Design of a High Efficiency Large Capacity Hydrogen Liquefier”; Adv. Cryog. Eng., Proc. CEC, Madison 2001, AIP, Vol. 613, 255-263) a hydrogen liquefier cycle that, to the inventors knowledge, most accurately represents the best current technology projections for hydrogen liquefaction cycles. It should be noted that Quack uses efficiency figures for compressors and turbines that are not achievable at present but which are thought to be realistic for the future.
Current hydrogen liquefaction processes consume power at a rate of about 11 kWh/kg(liquid hydrogen) based on a gaseous hydrogen feed at a typical pressure of 2.5 MPa (25 bar). Quack (“Conceptual Design of a High Efficiency Large Capacity Hydrogen Liquefier”; Adv. Cryog. Eng., Proc. CEC, Madison 2001, AIP, Vol. 613, 255-263) suggests that the best future power consumption will be in the range 5 to 7 kWh/kg(liquid hydrogen) if his suggested improvements are utilized.
This scheme involves pre-cooling the hydrogen to about −53° C. by indirect heat exchange with propane, ammonia, fluorocarbons or other refrigerants. The hydrogen is then further cooled and liquefied in two or more steps by indirect heat exchange against a mixture of helium and neon. The use of neon increases the molecular weight of the refrigerant mixture making it easier for the recycle compressor and thereby reducing compression energy (generally 75% He of MW=4 and 25% Ne of MW=20 having a mixture of MW=8). However, the use of neon in the mixture also prevents the temperature level of the refrigerant from achieving the very cold temperatures (−252° C.) required for the liquefaction of hydrogen. In addition, helium and neon must be sourced and its composition in the neon/helium mixture carefully managed. Also, unlike the present invention, this refrigerant must be compressed specifically and solely for the purpose of the hydrogen liquefaction energy.
It is a object of the present invention to develop a scheme which does not require neon or helium and which can efficiently provide refrigeration at the very cold end of the hydrogen liquefier (−252° C.).
The low molecular weight of hydrogen means that a very large number of stages of compression must be used, and if centrifugal compressors are specified and, likewise for a given expansion duty, a large number of centrifugal expansion stages will be required operating in series. In current plants, the hydrogen compressors are often reciprocating units. Multiple stage compressors plus large recuperative heat exchangers will usually be used in the pre-cooling steps of the process to remove the heat of compression.
The typical processes described above will typically utilize three hydrogen compressors and two nitrogen compressors (H2 Inlet Feed Compressor, H2 Cycle Feed Compressor, H2 Recycle Compressor, N2 Feed Compressor and N2 Recycle Compressor). As such, these types of systems and methods require significant capital and operational costs (e.g., both utilities and maintenance) in order to achieve the compression needed for the nitrogen and hydrogen refrigeration cycles.
Moreover, due to its low molecular weight and small molecular size, hydrogen is very difficult and therefore expensive to compress. Therefore, it is desired to produce liquid hydrogen without hydrogen compression or with reduced hydrogen compression lowering or reducing the associated large capital expenditures or large amounts of operational costs.